The explosive growth of digital communications technology has resulted in an ever-increasing demand for bandwidth for communicating digital data. Because of the scarcity of available bandwidth for accommodating additional digital communications, the industry recently turned its focus to the idea of utilizing the preexisting analog FM band more efficiently to help make such an accommodation. However, it is required that any adjustment to the FM band to provide the additional capacity for digital communications does not significantly affect the performance of the analog FM communications.
The above requirement is fully met by using a scheme disclosed in the co-pending, commonly assigned United States patent application of H. C. Papadopoulos and C. W. Sundberg entitled "Technique for Simultaneous Communications of Analog Frequency-Modulated and Digitally Modulated Signals using Precanceling Scheme," Ser. No. 08/704,470, filed Aug. 22, 1996. In accordance with this scheme, digitally modulated carriers are inserted adaptively in the FM band, without causing significant co-channel interference to a host FM signal in a simulcast of the FM signal and carriers. To that end, multiple carrier frequencies (or tones) for digital communications are generated which populate the FM band in a time varying manner. That is, the number of inserted carriers varies with time. The insertion process involves selecting as large a subset of the carrier frequencies as possible during a transmission interval to transmit digital data while the total degradation occasioned thereby to the host analog FM signal is kept at an acceptable level. The effect of precancellation (explained below) of each selected carrier modulated by digital data on the FM signal is taken into consideration.
Specifically, the above insertion process includes a carrier ranking subprocess, which involves an interference assessment in an emulated transmission of the FM signal with each carrier modulated by digital data. The carriers are ranked from the least interfering with the FM signal to the most interfering, and are selected in that order for digital data transmission up to a certain number corresponding to the acceptable limit of degradation to the FM signal. In addition, control information identifying the selected carriers is transmitted to the receiver through a control channel outside the FM band.
However, to ensure the integrity of the transmitted digital data, the interference by the analog FM signal to the digitally modulated carriers needs to be accounted for as well. According to the further aspects of the Papadopoulos and Sundberg scheme, since the waveform of the FM signal is a priori known at the transmitter, such interference can be precanceled by eliminating from the digitally modulated carriers the would-be effect of the FM signal thereon before the simulcast.